Solubility of Ionic Compounds | Chemical Bonding & s-Block

Solubility of Ionic Compounds

Thermodynamics & Periodic Trends | Chemical Bonding

1. Mechanism of Solubility

For an ionic compound to dissolve in a polar solvent like water ("Like dissolves Like"), the energy released during hydration must overcome the energy holding the crystal lattice together.

$$ \Delta H_{solution} = \Delta H_{Lattice} + \Delta H_{Hydration} $$

Lattice Energy ($U$): Energy required to break 1 mole of crystal into ions (Positive/Endothermic).
Hydration Energy ($\Delta H_{hyd}$): Energy released when ions interact with water (Negative/Exothermic).

                    Condition for Solubility:

                    Magnitude of Hydration Energy > Lattice Energy

2. Factors Affecting Solubility

A. Dielectric Constant of Solvent

Water has a high dielectric constant ($\epsilon \approx 80$), which reduces the electrostatic force of attraction between cation and anion by 80 times ($F_{new} = F_{air}/80$).

B. Size of Ions (Periodic Trends)

The trends depend on the relative sizes of the cation and anion.

Case 1: Salts with Small Anions ($OH^-, F^-, O^{2-}$)

Lattice energy depends inversely on ($r_+ + r_-$). Since the anion is small, increasing the cation size (down the group) causes a rapid decrease in Lattice Energy. Hydration energy decreases more slowly.

                    Trend: Solubility INCREASES down the group.

                    Ex: $Be(OH)_2 < Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$

Case 2: Salts with Large Anions ($SO_4^{2-}, CO_3^{2-}, NO_3^-, I^-$)

Large anions stabilize large cations (Efficient packing). Lattice energy does not decrease significantly down the group. However, Hydration energy decreases rapidly as cation size increases.

                    Trend: Solubility DECREASES down the group.

                    Ex: $BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$

3. Fajan's Rule & Solubility

Higher Covalent Character $\rightarrow$ Lower Solubility in Water.

$AgF$ (Ionic) $\rightarrow$ Soluble.
$AgCl, AgBr, AgI$ (Increasing Covalent character due to polarization) $\rightarrow$ Insoluble.

4. General Solubility Rules (Cheat Sheet)

Always Soluble	Generally Insoluble
All Nitrates ($NO_3^-$)	Carbonates (except Group 1 & $NH_4^+$)
Alkali Metals ($Li^+, Na^+, K^+ \dots$)	Phosphates (except Group 1 & $NH_4^+$)
Ammonium Salts ($NH_4^+$)	Hydroxides (except Group 1, $Ba^{2+}, Sr^{2+}$)
Acetates ($CH_3COO^-$)	Sulphides (except Group 1, 2)

Practice Quiz

Test your knowledge on Solubility Trends.

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